Wireless communication systems generally include control terminals (e.g. base stations or satellites) and mobile terminals (e.g. phones or radios). The information which is communicated between control terminals and mobile terminals includes not only user messages, such as speech or data, but also control messages which enable the control terminals to monitor and control the mobile terminals. In some communication systems, control messages can be communicated on the same radio frequency (RF) channel as user messages. In the digital cellular radio system standard known as Global System for Mobile Communication (GSM) and the land mobile radio standard known as Enhanced Digital Access Communication System (EDACS), developed by Ericsson, control messages can be transmitted as so-called Fast Associated Control Channel (FACCH) messages. FACCH messages generally carry urgent control information such as a handoff command where the control terminal instructs the mobile terminal to switch to another frequency or timeslot. The FACCH messages are assigned a higher priority than user messages so that they are transmitted across a channel before any awaiting user messages.
Since FACCH messages are transmitted on the same channel as user messages, a receiver must be able to reliably distinguish between FACCH and user messages. The different message types can be distinguished through flag bits which are transmitted with each message and serve to indicate the message type. However, flag bits increase the length of the transmitted messages and can become corrupted, thereby causing misidentification of associated messages.
An alternative approach to distinguishing FACCH and user messages is to transmit FACCH messages using one convolution rate and user messages using another different convolution rate. Consequently, the convolution rate of the transmitted message is indicative of whether the message is a FACCH message or a user message. A receiver determines which convolution rate was used to encode the transmitted message by decoding a received message using both convolution rates and comparing the resulting bit error rates (BERs) of both decoded messages. The decoding convolution rate which provides the lowest bit error rate corresponds to the encoding convolution rate of the transmitted message. Whether the message is a FACCH message or a user message is determined from the identified convolution rate of the transmitted message.
In practice, in noisy conditions where many of the binary values of a message can become reversed and where FACCH messages can have a relatively low number of logical "ones", there is a risk of a receiver erroneously identifying a FACCH message as a user message. Misidentification of a FACCH message results in the receiver failing to extract and use the FACCH information and, consequently, results in degraded performance of the communication system.